Orthogonal frequency division multiplexing (OFDM) has gained considerable interest by the research community and industry due to its highly desirable features for wireless transmission. Consequently, OFDM has been considered for several applications and standards, such as Wireless Local Area Network (WLAN, IEEE 802.11a and IEEE 802.11n), Worldwide Interoperability for Microwave Access (WiMax, IEEE 802.16), and Mobile Broadband Wireless Access (MBWA, IEEE 802.20). Although OFDM is highly robust against various transmission impairments, it does not have any inherent security features. Hence, additional encryption/decryption algorithms should be implemented for data security.
One type of secure communication algorithm is a block cipher. The main building parts of a block cipher are a round function based on a nonlinear operation, mixing component and round keys. If the number of rounds is more than 32, breaking this cipher will be very difficult. The main limitation of commonly designed ciphers is their low speed, which is a major drawback for today's broadband systems.
Digital encryption is usually applied to the transmitted bits at the data link layer or at higher protocol layers of the communication protocol stack. Block encryption techniques permute blocks of bits in a key-dependent way, while stream ciphers first generate a key-dependent pseudo-random binary key stream, which is then XOR-ed with the plaintext bit sequence to produce the cipher text. An eavesdropper without access to the key cannot easily recover the plaintext from an intercepted cipher text.
The Data Encryption Standard (DES) is classified as a block cipher and has been used worldwide in the banking communities and for electronic fund transfers. Due to concerns about security of DES—such as a short key size (e.g., 56 bits), slow operation, and differential and liner cryptanalysis—security communities have sought to replace DES with more robust algorithm. Therefore, a new symmetric key cryptosystem, the Advanced Encryption Standard (AES) was announced in 2001. AES is efficient in hardware and software implementations with various key sizes of 128, 192 and 256 bits.
Although there are several cryptosystems designed to operate at the lowest layers (physical layer) of the protocol stack for OFDM based systems, such techniques are usually designed to function at the bit or symbol level. For example, European Patent No. 1 513 279 B1 describes a system that encrypts the baseband QAM symbols by changing their phase according to a given key sequence before the inverse fast Fourier transform (IFFT) process. Moreover, the training symbols that are embedded for synchronization and channel estimation are encrypted as well. Consequently, the encryption process hides the necessary information required for synchronization and channel estimation, which are necessary to recover the encrypted data symbols. Furthermore, the data symbols themselves are encrypted as well. The main limitation of this approach is that it is suitable only for systems with training symbols. Future communication systems may not rely on pilot symbols for synchronization and channel estimation as several blind techniques have been proposed. Moreover, the performance of this technique mainly relies on channel conditions. If the channel is flat, it should not be difficult to estimate the channel parameters, even with encrypted pilots.
U.S. Pat. Nos. 7,751,488 and 7,649,951 describe a security system for OFDM by mixing the phases of the data symbols and varying the data-to-subcarrier assignment based on a secret key sequence. Therefore, an eavesdropper needs first to know the mapping between data and subcarriers, and then the phase/amplitude of the data symbols. Similar to the above mentioned systems, there are several other encryption systems that are based on the general concept of building an encryption technique by scrambling the frequency domain symbols (e.g., U.S. Patent Application Publication No. US 2011/0033051; A. Chorti and I. Kanaras, “Masked M-QAM OFDM: A simple approach for enhancing the security of OFDM systems,” IEEE 20th International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC), vol., no., pp. 1682-1686, 13-16 Sep. 2009; and M. Khan, M. Asim, V. Jeoti and R. Manzoor, “On secure OFDM system: Chaos based constellation scrambling,” International Conference on Intelligent and Advanced Systems (ICIAS), pp. 484-488, November 2007).
Unlike the approaches described in the previous paragraph, very little research has been conducted to perform encryption/decryption using the time-domain samples of OFDM signals. However, U.S. Pat. No. 6,650,616 describes introducing intentional group delay to one or more subcarriers at the transmitter using a series of filters. The group delay is supposed to destroy the frequency orthogonality of the signal and hence prevent correct data detection. In principle, there are a very large number of different group delays that may be applied. Therefore, it should be infeasible for an eavesdropper to ascertain the introduced group delay. However, the main limitation of this system is the high computational complexity as the number of divisions and multiplications to encrypt each OFDM symbol is quite large.